WASHINGTON - The cost of buying and operating a new fleet of jet fighters for the U.S. military is nearing $1 trillion, according to a congressional audit that found the program dogged by delays, manufacturing inefficiencies and price increases.

Released March 12, the report from the Government Accountability Office offers a sobering assessment of the ambitious effort to deliver a modern series of aircraft known as the F-35 Lightning II to the Navy, Air Force and Marine Corps.

Tasked by Congress to conduct an annual assessment of the program, the GAO said costs have gone up by $23 billion since last year alone.

Close to $300 billion is needed to acquire 2,458 aircraft for the three services and another $650 billion will be needed to operate and maintain the fighters that are expected to be flying well into the 21st century, the report says.

Operating costs, projected at $346 billion just a few years ago, have been driven upward by changes in repair plans, revised costs for depot maintenance, higher fuel costs and increased fuel consumption.

The GAO's auditors said they expect development and procurement costs "to increase substantially and schedule pressures to worsen based on performance to date."

Lockheed Martin Aeronautics Co. of Fort Worth, Texas, is the prime contractor for the Lightning II, also known as the Joint Strike Fighter.

The GAO, the investigative arm of Congress, also sees many of the problems as self-inflicted.

"The contractor has extended manufacturing schedules several times, but test aircraft delivery dates continue to slip," the report states. "The flight test program has barely begun, but faces substantial risks with reduced assets as design and manufacturing problems continue to cause delays that further compress the time available to complete development."

Auditors criticized both the military and the contractor for pressing into the jet's development's phase before key technologies were mature, started manufacturing test aircraft before designs were stable, and moved to production before flight tests showed the aircraft was ready.

"We do not know the basis for the GAO estimates and until we receive and analyze their data we will be unable to comment on them," Lockheed spokesman John Smith said in an e-mailed statement.

Smith, however, said the company has been careful stewards of U.S. tax dollars by trimming costs wherever possible.

"We continue to apply the same kind of oversight, budget alignment and lean thinking to the program," he said.

Production of the Lightning II has begun and the Defense Department is scheduled to buy the aircraft through 2034. U.S. allies are also buying hundreds of the jets and are contributing $4.8 billion in development costs.

The Lightning II is being produced in several different models tailored to the needs of each service. The new jet will replace the Air Forces F-16 Falcon and the A-10 Warthog aircraft. A short takeoff and vertical landing version will replace the Marine Corps F/A-18C/D and AV-8B Harrier aircraft. And the Navy is buying a model designed for taking off and landing on aircraft carriers.